Seismic Reliability of Structural Systems Isolated by FPS

The paper deals with the seismic reliability of structural systems equipped with friction pendulum isolators (FPS). The behavior of these systems is analysed by employing a two-degree-of-freedom model accounting for the superstructure flexibility, whereas the FPS device behaviour is described by adopting a widespread model which considers the variation of the friction coefficient with the velocity. The uncertainty in the seismic inputs is taken into account by considering a set of artificial records, obtained through the power spectral density method, with different characteristics depending on soil dynamic parameters, and scaled to increasing intensity levels. The friction coefficient at large velocity is considered as random variable modeled through a uniform probability density function. Within incremental dynamic analysis, Monte Carlo simulations are developed in order to evaluate the probabilities exceeding different limit states related to both superstructure and isolation level defining the seismic fragility curves through an extensive parametric study carried out for different structural system properties. Finally, considering the seismic hazard curve related to L’Aquila site (Italy), reliability-based abacuses and regression expressions are derived with the aim to design the radius in plan of the friction pendulum devices in function of the structural system properties and reliability level expected.